In recent years there has been much interest in mining deepsea ferromanganese crusts, as potentially economic ore bodies occur within exclusive economic zone (EEZ) areas of countries. These crusts are found on the tops and flanks of seamounts within a wide range of water depth. The crusts are significantly enriched in cobalt, manganese, nickel, titanium and platinum, and represent an important future source of strategic metals. Resource estimates indicate numerous seamounts in the U.S. Middle Pacific EEZ, with a minimum of 5 million tons of accessible crust on each with an average gross value of $510.00 per ton at 1983 metal prices.
Previous ferromanganese crust mining systems require mechanical breakage of the crust by rip teeth on dredge buckets or by various designs of cutters or ultrasonic disintegrators on a bottom crawling mining machine. Physical transport of the ore material to the surface is required by means of continuous line buckets, slurry air lift or slurry hydraulic lift. The ore material is dewatered at the surface by a mining support ship and transported ashore for pyrometallurgical smelting or hydrometallurgical leaching processing and refining.
Solution mining is a relatively recent advent in the history of mining. Modern, large-scale hydrometallurgical processing had to wait for the introduction of electrical power needed for electrolytic reduction and power for transport of large volumes of slurries and solutions. In the past 30 years, solution mining had increased to account for 18% of the U.S. annual copper production, which peaked in 1978 prior to the collapse of the copper industry. Innovations in concentration processes during the past decade have led to significant production of gold, silver and uranium, most notably in conjunction with the use of solution mining heap leach techniques on mine tailing heaps.
There are a number of variations in process and procedure in hydrometallurgical ore leaching practice, dependent on ore composition and grade. Hydrometallurgy in situ leaching is used in the following processes:
Sulfuric acid leach solution (barren leach liquor) is directly applied to the ore body. PA1 Metals present in the ore such as cobalt, manganese, iron, nickel and copper are extracted as soluble sulfates and are drawn off as a pregnant leach liquor. PA1 Metals are selectively separated from the pregnant leach liquor by different conventional processes.
For example, cobalt is separated and purified by extraction from the pregnant leach liquor by a highly selective cobalt-to-iron exchange solvent or resin such as Amberlite DPL forming a concentrated liquor. The cobalt can then be recovered from the concentrated liquor by precipitation by hydrogen gas reduction or by electrowinning.
Processed pregnant leach liquor and concentrated liquors are reconstituted and recycled as barren leach liquor.
A number of extractive metallurgy processes potentially suitable for hydrometallurgical mining of ferromanganese ore have been investigated. These include ammoniacal leach with ferrous sulfate reductant, cuprion ammoniacal leach, hypochlorous acid leach, and sulfuric acid leach. Experimental extraction results for each are favorable compared to traditional smelting process results. However, leach kinetics at the in situ temperatures and pressures of submarine ferromanganese crusts were not considered or evaluated. Further research and development of metallurgy leach processes for submarine solution mining is necessary.